[System.Serializable]
public class UsersClass
{
public int _users_id;
public string _users_username;
public string _users_password;
public string _users_email;
public string _users_name;
public string _users_surname;
public string _users_adress;
public string _users_zipCode;
public string _users_phone;
public string _users_picture;
public string _users_date_login;
public string _users_date_registration;
public string _customers_group_customers_group_name;
public string _country_country_name;
public string _users_type_type_name;

The variable names in UsersClass are badly named. They do not match with what the server is sending you. Please remove _ from the beginning of each variable name in the UsersClass class.

For example, you named the username_users_username instead of users_username. All the variables are affected, not just one.

To make sure you got all the spellings right, go here and paste the Json file you received from the server. It will give you a complete class and variable names. Copy those variables and replace them with what you have now. Just make sure to remove { get; set; } from each variable. Unity Json can't work with that.

Also stop naming your classes _classname. The first letter should be capitalized and not an underscore.